This invention relates to back tension imparting devices, and more particularly it is concerned with a device for imparting back tension to a strip of photoconductive film wound on a photosensitive drum for an electrophotographic copying apparatus.
In an electrophotographic copying apparatus of the transfer printing type, an intermediate image is formed on a photoreceptor from an electrostatic latent image of an original to be copied or a toner image obtained by causing toner particles to adhere to the electrostatic latent image and is then, duplicated by transfer printing on a sheet of ordinary paper and developed or fixed so as to produce a copy of the original.
In one type of apparatus, such photoreceptor may be deposited by vacuum vaporization of zinc oxide on the outer peripheral surface of a drum. This type of photoreceptor has the disadvantage of being expensive and it is troublesome to replace the old photoreceptor by a new one when the former deteriorates after repeated use for production of copies. In order to obviate these disadvantages proposals have been made to use a photoreceptor in flexible film form which is wound on the outer peripheral surface of a drum and affixed at opposite ends to a supply reel and a take-up reel mounted in the interior of the drum. When it is desired to replace the portion of the photosensitive film on the outer peripheral surface of the drum by another portion, the photosensitive film can be paid out of the supply reed by rotating the take-up reel and thus the old portion of the photosensitive film serving as a photoreceptor can be replaced by a new portion with ease.
It is necessary to impart back tension of a predetermined magnitude to the photosensitive film wound on the other peripheral surface of the drum in order that the film may be maintained in intimate contact with the drum since the film may become loose as one portion is replaced by a new portion thereof by rotating the take-up reel or when copying is performed repeatedly. When loosening of the photosensitive film occurs, many troubles will occur as presently to be described. In the charging station, the distance between the charging electrode and the photosensitive film or photoreceptor on the drum will become non-uniform and thus uneven charging of the photorecptor or destruction thereof by charging will result. In the developing station, the distance between the opposite electrode and the photoreceptor will become non-uniform and thus it will become impossible to produce copies with developed images of uniform quality, and the contact of the photoreceptor with the opposite electrode will cause a leak of the bias potential and soiling of the background of the image. If the toner particles find their way to the underside of the photoreceptor and adhere to the outer peripheral surface of the drum, the photoreceptor will become irregular in surface and tend to be damaged in the cleaning operation. Besides, there will be a decline in the resolving power of the photoreceptor. When a developing agent of the wet type is employed, some of the developing liquid will find its way to the underside of the photoreceptor. Such liquid will move about on the outer peripheral surface of the drum when pressure is applied to the photoreceptor by the cleaning or transfer-printing roller, and will finally be scattered transversely in the passageway of the photoreceptor in and out of the drum or introduced into the interior of the drum.
In an effort to prevent loosening of the portion of the photosensitive film serving as a photoreceptor on the outer peripheral surface of the drum, proposals have been made to apply a brake to the supply reel from which the photosensitive film is paid out. This has, however, had no effect of preventing loosening of the photosensitive film and the aforementioned troubles have therefore been experienced. Also, the use of a floating roller to prevent the loosening of the photosensitive film has been proposed. This has the following disadvantages. The floating roller needs a brake to form a combination therewith and thus the mechanism becomes complex in construction and takes more space than is necessary. Adjustments of the mechanism are troublesome because the brake should be applied to the supply reel with a force which is higher than the drive force exerted by the floating roller and lower than the force with which winding of the film is effected by the take-up roller. When the brake force is set at a level such as to ensure that the supply reel is not rotated by the floating roller, the brake force is considerably high and the torque required for advancing and winding the film on the take-up reel becomes high, thereby increasing possibilities of damage to the film serving as a photoreceptor and increasing the magnitude of the load applied to each part of the apparatus.